I wouldn't follow what Adafruit do, they often get things wrong. The answer on stackexchange is incorrect, thermal relief is only needed if you are using a point heat source (ie. soldering iron). During reflow all of the board is heated, so the answer is nonsense. Probably the real reason is to confine the solder to the central square. There is some good advice later on about surface finishes.
I would start again with the manufacturer's data sheet and not worry about some weird stuff someone else has done.
Anyway, you can overlap multiple pads, but creating slots like that might be tricky.